Method for generating a channel quality indicator by biasing signal-to-interference ratio

ABSTRACT

A method for biasing signal-to-interference ratio (SIR) to generate channel quality indicator (CQI) includes measuring the packet error rate (PER) of a received signal and comparing the PER to a to the target PER to generate a correction term. The correction term is combined with the SIR estimation of a reference channel to generate a CQI. The CQI is reported to a transmitter to adjust signal configurations, such as code rate, modulation type, number of codes, power offset.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. ProvisionalApplication No. 60/482,809 filed on Jun. 26, 2003, which is incorporatedby reference as if fully set forth.

FIELD OF INVENTION

[0002] This invention relates to generation of a channel qualityindicator (CQI) in wireless communications, and more particularly to amethod for adaptive biasing of the signal-to-interference ratio (SIR)for CQI generation.

BACKGROUND

[0003] Adaptive Coding and Modulation (ACM) is an effective techniquefor providing link adaptation in both Uplink (UL) and Downlink (DL)communications. ACM is typically accomplished by algorithms workingtogether in both the receiver and the transmitter. The receiver makes anestimate of the channel quality by measuring the SIR of one or moretransmissions from the transmitter. The measured SIR is then used tocompute a CQI.

[0004] Each CQI may correspond to a particular configuration of radioresources such as code rate and modulation type. After each SIRmeasurement is made, the CQI is computed. For example, the SIR iscompared to a table of SIR-CQI pairs and the CQI value that yields thebest performance, such as in terms of Packet Error Rate (PER) orthroughput, is selected and sent back to the transmitter. Thetransmitter then selects a radio configuration that is no moreaggressive than that indicated by the received CQI value. In anotherwise unpopulated cell, the transmitter would simply use aconfiguration consistent with the channel quality indicated by the CQI.

[0005] However, SIR alone does not give complete information about thequality of the channel. For example, in order to maximize datathroughput, a channel with a large delay spread and/or a large Dopplerspread should not be assigned as high of a CQI value. Additionally, theCQI will depend on the receiver design.

SUMMARY

[0006] The present invention provides a system and method for biasingthe SIR value that is used for CQI generation in order to maintain acertain specified packet error rate (PER) as a mechanism to maximizedata throughput. In this way, the proper CQI values are selectedregardless of the type of propagation channel. Furthermore, additionalalgorithms such as Doppler estimators, or other feed forward correctionterms, are not required but may be used in conjunction with theinvention to further improve throughput performance.

[0007] In a preferred embodiment relating to Frequency Division DuplexHigh Speed Downlink Packet Access (FDD-HSDPA), a CQI table is specifiedsuch that each entry in the table corresponds to the code rate,modulation type, number of codes, and power offset that differ by 1 dBin required Common Pilot Channel (CPICH) SIR to the target PER. The CQIvalue is derived by adding a bias to the estimated CPICH SIR.

[0008] In the prior art, the bias was fixed and was determined primarilyby the design of the table, the signaled power ratio of the pilot signaland the data signal, and the desired performance over a large class ofchannel types. By employing the present invention, the need to select acompromised fixed bias is removed. This is accomplished by monitoringthe performance in terms of PER and adjusting the SIR bias to maintain adesired long-term average PER performance.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0009]FIG. 1 is a block diagram of a system for generating CQI inaccordance with the present invention.

[0010]FIG. 2 is a block diagram of a system for generating a CQI valueby biasing SIR with a correction term created from the ACK/NACK signalaccording to a preferred embodiment of the present invention.

[0011]FIG. 3 is a block diagram of a system for generating a CQI valuewith several channel-type correction terms and a SIR prediction term inaccordance with an alternative embodiment of the present invention.

[0012]FIG. 4 is a signal diagram of the result of simulation forlong-term PER for ITU channels under various levels of inter- andintra-cell interference.

[0013]FIG. 5 is a signal diagram of the results of simulation of theACK/NACK signal and adaptive bias correction signal generated accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0014] The present invention will be described with reference to thedrawing figures wherein like numerals represent like elementsthroughout.

[0015] The present invention is generally applicable to all modes of theThird Generation (3G) standards including Time Division Duplex (TDD),Frequency Division Duplex (FDD), Time Division Synchronous Code DivisionMultiple Access (TDSCMA) and Code Division Multiple Access 2000 (CDMA2000) scenarios, but is envisaged to be applicable to other scenarios aswell.

[0016]FIG. 1 is a block diagram of a system 8 for signal processing inaccordance with the present invention. A data packet transmitted from atransmitter (not shown) is received by a receiver, only a portion ofwhich is shown in FIG. 1. SIR estimator 20 calculates an SIR estimate ofa reference channel based on the received data packet by a conventionalmethod. The received data packet is also input into a correction termgenerator 10 and processed for error detection. An ACK/NACK signalgenerated from the error detection process is used to generate acorrection term to bias the SIR estimate. The SIR estimate and thecorrection term are combined at the combiner 21 to generate a correctedSIR.

[0017] The corrected SIR is mapped by the SIR-CQI map 30 to generate aCQI value. The CQI value is reported to the transmitter in order toadjust the configuration of the next transmitted data packet. The CQIvalue ensures desired throughput performance by maintaining a PER thatis close to the target PER. The purpose of maintaining a PER that isclose to the target PER is that it optimizes the utilization of radioresources and, therefore, the data throughput of the system.

[0018]FIG. 2 is a block diagram of a system 5 for generating acorrection term and CQI from the corrected SIR according to the presentinvention. A received data packet is entered into SIR estimator 20 andan SIR estimate of a reference channel is generated based on thereceived data packet. This is accomplished by a conventional SIRgeneration method which will not be further described.

[0019] The received data packet is also input into the adaptivecorrection term generator 10. The adaptive correction term generator 10detects whether an error exists within the received data packet. Theoccurrence of the error is detected by cyclic redundancy check (CRC)unit 11. CRC unit 11 generates an acknowledgement (ACK) signal if noerror has been detected or a negative-acknowledgement (NACK) signal ifan error has been detected. These ACK/NACK signals resulting from thecomputation of the CRC are mapped into ‘0s’ and ‘1s’ respectively by aprocessor 12. This signal is then preferably filtered by a filter 13 toprovide a better long-term estimate of the PER of the current channel.It should, however, be understood that the filter 13 is not required. Asimple Infinite Impulse Response (IIR) filter with a slowly decayingexponential impulse response can be used, but other filters may also beused.

[0020] The adaptive correction term generator 10 also includes aprogrammable target PER unit 16. The target PER unit 16 generates atarget PER to ensure the desired performance, such as maximizingthroughput. The required PER is defined as the PER required to properlydecode a packet with a predetermined probability.

[0021] The filtered binary ACK/NACK signal generated from the processor12 is combined by a summer 14 with the target PER to generate an errorsignal that represents a deviation of current performance of thereceiver from the target performance of the receiver in terms of PER.

[0022] A Proportional Integral Derivative (PID) unit, or other unitcontaining higher order linear or non-linear elements, processes theerror signal. Simulations have shown that a proportional term aloneprovides some correction, and the combination of proportional andintegral terms is sufficient to nearly completely correct the CQIestimates in terms of average PER requirements.

[0023] The value output by the PID unit 15 is added to the SIR estimateby a summer 32 to generate the corrected SIR value. The corrected SIRmay be filtered further by a filter 31, if additional processing isdesired. This filter 31 may include non-linear elements such as limitersand slew-rate limiters. The result is processed to generate proper CQIvalue through a SIR-CQI mapping unit 30. The mapping unit 30 maycomprise a look-up table or other type of functional relationship whichgenerates a predetermined value for CQI according to the input correctedSIR.

[0024] The CQI value is transmitted to the transmitter to adjust thesignal configuration to obtain the best performance in terms of PERunder the current channel conditions. The signal configuration that thetransmitter may adjust includes code rate, modulation type, number ofcodes and power offset according to the CQI.

[0025] An alternative embodiment of the present invention shown in FIG.3 is a system 102 having a channel-type correction term generator 40 anda SIR prediction term generator 50 to generate additional correctionterms that can be used to bias and correct the SIR estimate in additionto the correction term generated as described with reference to FIG. 2.These terms may include delay spread corrections 41, Doppler spreadcorrections 42, battery voltage dependant losses introduced by theradio, or other channel-type corrections 43. As previously indicated,SIR alone does not completely define the quality of the channel. Largedelay spreads and Doppler spreads have the potential to degradeperformance of the receiver. Therefore, knowledge of these quantitiescan be used to improve the bias applied to the SIR, or alternatively toadjust the target PER.

[0026] The SIR prediction term generator 50 may also be employed toimprove the bias by estimating what the SIR will be at a time (in thefuture) when the transmitter will transmit the next packet. Processingby the derivative of the SIR (d/dt) 55 leads to a very simple form ofpredictor, while other more complex predictors may be further used. Thegenerated predictive terms may be processed by clipping, dead zoning orany other non-linear processing techniques 57.

[0027] It should be noted that the channel-type correction termgenerator 40 and the SIR prediction term generator 50 may be used aloneor in combination with each other or with the adaptive correction termgenerator 100 to generate an SIR correction. These terms can begenerated by conventional methods. Accordingly, these methods will notbe further described hereinafter. These terms are used to correct theSIR value based on short term measurements of the channel type, and maybe used in conjunction with the adaptive SIR biasing scheme of thepresent invention. The correction terms from the channel-type correctionterm generator 40 and from the SIR prediction term generator 50 aresummed via a summer 52. The resulting composite error correction term isadded by the summer 32 to generate a corrected SIR. The term “correctedSIR” has been used throughout to indicate an effective SIR that betterconveys the net losses due to the channel, with its various parameters,and receiver losses.

[0028]FIG. 4 is a signal diagram of the results of simulation forlong-term PER for several ITU channels, which are defined in 3GPPstandard for performance verification under various levels of inter- andintra-cell interference. The channels are Pedestrian A 3 kmph (PA3),Pedestrian B 3 kmph (PB3), and Vehicular A 30 kmph (VA30). Each channelwas simulated with all combinations of Ec/Ior=−3 and −6 dB andIor/Ioc=0, 5, and 10 dB (6 points in all) for 10,000 packets.Measurements of PER over the simulations in FIG. 4 show that the averageresulting PER is maintained near the target PER for a wide class ofchannel types, intra-cell interference levels, and inter-cellinterference levels.

[0029]FIG. 5 is a signal diagram of the results of simulation for theconvergence of the adaptive bias term and the corresponding ACK/NACKsignal in ITU PB3 fading channel. Notice that initially only NACKs (CRCfailures) are observed. After the adaptive bias correction term of thepresent invention has biased the SIR estimate for CQI generation, ACKs(CRC successes) are more frequent. The average PER is maintained nearthe target PER after convergence, about 200 packets or about 0.4 sec inFDD-HSDPA, much faster than is expected to be required.

[0030] While this invention has been particularly shown and describedwith reference to preferred embodiments, it will be understood by thoseskilled in the art that various changes in forms and details may be madetherein without departing from the scope of the invention as describedabove.

What is claimed is:
 1. A method for generating a channel qualityindication for communication channel between a transmitter and areceiver, the method comprising the steps of: (a) measuringsignal-to-interference ratio from a received data packet; (b) generatinga channel quality correction term from an error detection process forthe received data packet; and, (c) calculating a channel qualityindication by combining the results of steps (a) and (b).
 2. The methodof claim 1 further comprising the steps of: generating a correction termfor Doppler spreading; and combining said Doppler correction term withthe results of steps (a) and (b) to calculate the channel qualityindication at step (c).
 3. The method of claim 1 further comprising thesteps of: generating a correction term for delay spreading; andcombining said delay correction term with the results of step (a) and(b) to calculate the channel quality indication at step (c).
 4. Themethod of claim 1 wherein the error detection process is a cyclicredundancy check.
 5. The method of claim 1 wherein step (b) comprisesthe steps of: detecting an error on the received data packet; generatingan ACK/NACK signal according to the error detection result; generatingan error signal by subtracting a target PER from the ACK/NACK signal;and generating the correction term from the error signal.
 6. A systemfor adaptive control of transmit signal configuration according tochannel conditions between a transmitter and a receiver; the transmitterwhich adjusts signal configuration in accordance with a channel qualityindication (CQI); and the receiver comprising: a signal-to-interference(SIR) estimator for calculating the SIR of a received data packet; anerror detection unit for generating a correction term from the receiveddata packet; and, a combiner for combining an SIR estimate with thecorrection term to generate a corrected SIR; and, a mapping unit forgenerating said CQI.
 7. The system of claim 6 further comprising afilter for filtering the result of error detection unit.
 8. The systemof claim 6 wherein the error detection unit performs a cyclic redundancycheck.
 9. The system of claim 6 further comprising a channel qualitycorrection term generator to generate a channel quality correction termfor input into said combiner.
 10. The system of claim 9 wherein saidchannel quality correction term is based upon Doppler spreading.
 11. Thesystem of claim 9 wherein said channel quality correction term is basedupon delay spreading.
 12. A method for adaptively adjusting signalconfiguration in accordance with a channel quality indication, themethod comprising the steps of: (a) measuring the signal-to-interferenceratio of a received data packet; (b) generating a channel qualitycorrection term from error detection process of the received datapacket; (c) calculating a channel quality indication by combining theresults of steps (a) and (b); and, (d) adjusting the signalconfiguration according to the channel quality indication.
 13. Themethod of claim 12 wherein the signal configuration includes code rate,modulation type, number of codes and power offset.
 14. The method ofclaim 12 further comprising the step of generating a channel qualitycorrection term from Doppler spreading to combine with the result atstep (c).
 15. The method of claim 12 further comprising the step ofgenerating a channel quality correction term from delay spreading tocombine with the result at step (c).